Film resistance unit



Nov. 29, 1966 c. M. HYDE 3,289,139

FILM RESISTANCE UNIT Filed Feb. 24, 1964 United States Patent 3,289,139 FILM RESESTANQE UNIT Clyde M. Hyde, Lincoln, Nebr., assignor to Dale Electronics, ind, Columbus, Nehru, a corporation of Nebraska Filed Feb. 24, 1964, Ser. No. 346,615 9 Claims. (Ci. 338-218) Film-type resistors are usually comprised of a cylindrical core of non-conducting material upon which a thin coating of electro-conducting resistive material is sprayed or otherwise applied. Terminal caps are sometimes then placed on the ends of the core and in electrical contact with the resistive film to permit the film to be connected in an electrical circuit.

The resistive-coated core is usually placed in a spindletype lathe unit after the terminal caps are mounted, and a cutting tool is moved along the length of the core as the core is being rotated to score or cut the resistive film so that the resultant film extends in a helical-shaped ribbon over the outer surface of the core. Such resistors develop heat substantially uniformly along the length of the ribbon of constant width, but because the heat is more easily and quickly dissipated from the ends of the unit, the heat build-up near the center of the resistor becomes substantial. The creation of excessive temperatures near the center of the resistor obviously affects its efficiency and capacity. This is a problem that is inherent in resistors of various designs.

A further disadvantage of the above-described resistors is the development of hot spots at points on the ribbon. These hot spots generally occur adjacent the terminal ends of the groove cut in the film, and are caused by the increased heat which results from a concentration of current lines as they pass around the turn created in the ribbon by the diagonally disposed terminal ends of the groove.

Therefore, a principal object of this invention is to provide a film resistor unit that will prevent the creation of hot spots in the resistor film adjacent the terminal ends of the groove scored in the film.

A further object of this invention is to provide a film resistor unit which will create less heat at its center portion than at the ends thereof to prevent an excessive temperature build-up at the center portion.

A still further object of this invention is to provide a film resistor which can have its resistance pattern determined by the relative widths of the film ribbon convolutions appearing on the surface of the resistance core.

A still further object of this invention is to provide a film resistor that is economical to manufacture, and durable in use.

These and other objects will be apparent to those skilled in the art.

This invention consists in the construction, arrangements, and combination, of the various parts of the device, whereby the objects contemplated are attained as hereinafter more fully set forth, specifically pointed out in the claims, and illustrated in the accompanying drawing, in which:

FIGURE 1 is a side elevational view of the resistor of this invention with the terminal caps being shown in cross section to more fully illustrate the construction of the resistor;

FIGURE 2 is a sectional view of the resistor taken on line 22 of FIGURE 1; and

FIGURE 3 is a perspective view of the completed and assembled resistor.

The resistor includes an elongated cylindrical core 12 of any desired non-conductive ceramic or similar material. An electrical resistance film 14 of a tin oxide Patented Nov. 29, 1966 "ice composition or the like is sprayed or otherwise placed on the outer surface of the core and adheres thereto through its own adhesive properties. The film is allowed to dry and the core is then placed in a lathe device wherein the rotational speed of the resistor and the rectilinear speed of the cutting tool can be precisely controlled. A helical groove 16 is then cut through the film from one end of the unit to the other. The mechanics of creating this groove are not involved in the instant invention, and the groove can be created in a variety of ways. However, it is important that the terminal ends 16a and 16b of the groove be arcuate in shape. As shown in FIG- URE 1, the extreme ends of the groove progress inwardly from a substantially horizontal direction through an arcuate path to a tangential junction with the main body of the groove.

As will be noted in FIGURE 1, the groove 16 is symmetrically cut with respect to the center transverse axis of the core 12, and each convolution of the helical groove is progressively narrowed or shortened in pitch, width or depth as the groove extends towards the end of the unit from this center axis. This disposition of groove 16 divides the film into helical convolutions of varying Widths, with the convolution 18 being substantially symmetrically disposed on the transverse center axis of the core, and the identical pairs of convolutions 20, 22, 24, 26, 28, 3t), 32 and 34 being formed on either side thereof. The convolutions 20 through 32 progressively decrease in their lateral widths both within the individual convolution, and from convolution to convolution.

If terminal caps 36 and 37 of electro-conducting material are to be used, they are secured to the ends of the film-covered core 12 in any convenient manner. However, in no event should they extend inwardly beyond end portion 16a of groove 16. Groove 16 is normally cut after the caps are so mounted. Terminal rods 38 are concentrically mounted on the caps and extend outwardly therefrom in conventional fashion.

As electrical power proceeds to the resistor through its connection in an electrical circuit from cap 36 into film 14, the lines of current, because of the arcuate end portion 16a of groove 16, follow a gently curving path from the end of the film downwardly into the convolution 34- at that end of the resistor. Had the end 1611 of the groove been abruptly terminated instead of bent outwardly on a curved path towards the end of the core, the lines of current would have been sharply bent and converged upon each other as they passed downwardly into the initial convolution of the film 14. This convergence of current lines around such a sharp bend creates an excessive temperature, for the heat power generated is equal to the product of the resistance and the square of the current. Obviously, by converging the current lines and perhaps doubling their intensity, the resuiting amount of heat generated is quadrupled. By preventing this concentration of current lines around a sharp bend through directing these lines around the gentle arcuate path above the terminal groove portion 16a, this concentration of heat is avoided. The same phenomenon takes place at the opposite end of the resistor in regard to the current lines passing by terminal groove portion 1612.

Since the resistance of any electrical conductor of uniform thickness is inversely proportional to the width thereof, the resistance of the wider central-most convolutions of the film ribbon creates less heat than the narrower convolutions at the end of the resistor, as computed according to the formula, Heat Power=I R where I is the amperage or current and R is the resistance of the section of ribbon being contemplated. Because the central portion of the resistor unit generates proportionately less heat than the ends thereof, a balance of heat level throughout the resistor is achieved because the greater heat on the ends thereof is more quickly dissipated than the smaller heat quantum at the center thereof. Obviously, the resistance performance of the unit can be controlled by changing the relative widths of the film ribbon convolutions according to any desired predetermined design.

Although an elongated cylindrical core has been shown on the drawings, a core of fiat construction or a core which is rectangular in cross-section could be made to operate satisfactorily with the above described groove 16. In addition, automation can permit the helically shaped groove 16 to be cut on the interior surface of a hollow core as well as on an exterior surface.

From the foregoing, it is seen that the film resistor of this invention will eliminate the heat build-up at the center thereof, and will avoid the creation of hot spots at the terminal ends of the scored groove. In addition, the resistance potential of such a resistor can be varied according to predetermined designs by controlling the varying widths of the film ribbon convolutions. The objectives of this invention are therefore seen to have been met by the resistor herein disclosed.

Some changes may be made in the construction and arrangement of my film resistance unit without departing from the real spirit and purpose of my invention, and it is my intention to cover by my claims, any modified forms of structure or use of mechanical equivalents which may be reasonabley included within their scope.

I claim:

1. In a resistor device,

an elongated core of non-electro-conducting material,

a resistance film on the outer surface of said core,

electro-conducting terminal means on the ends of said core in electrical contact with said film,

a helical groove being cut in said film to divide said film into a continuous ribbon comprised of a plurality of convolutions,

the lateral width of the convolutions near the center of said core being wider than the convolutions adja cent the end of said core,

the ends of said groove departing from a helical pattern and terminating in an arcuate shape and extending substantially horizontally and longitudinally towards the respective ends of said core.

2. The resistor of claim 1 wherein the convolutions of ribbon become progressively narrower as they extend outwardly towards the end of the core away from the center thereof.

3. The resistor of claim 1 wherein the shape of said convolutions of ribbon are substantially symmetrical ab out the center transverse axis of the core.

4. In a resistor device,

an elongated core of non-electro-conducting material,

a resistance film on the outer surface of said core,

electro-conducting terminal means on the ends of said core in electrical contact with said film,

a helical groove being cut in said film to divide said film 6 into a continuous ribbon comprised of a plurality of convolutions,

the ends of said groove departing from a helical pattern and terminating in an arcuate shape, and extending substantially horizontally and longitudinally towards the respective ends of said core.

In a resistor device,

a core element of non-electro-c0nducting material having an exposed surface area adapted to receive a resistance film,

a resistance film on said exposed surface area of said core element,

a groove being cut in said film to divide said film into a ribbon comprised of a plurality of convolutions,

the lateral width of the convolutions near the center of said core being wider than the convolutions adjacent the ends of said core,

the ends of said groove departing from a helical pattern and terminating in an arcuate shape and extending substantially horizontally and longitudinally towards the respective ends of said core.

6. The resistor of claim 5 wherein the convolutions of ribbon become progressively narrower as they extend outwardly towards the end of the core away from the center thereof.

7. The resistor of claim 5 wherein the shape of said convolutions of ribbon are substantially symmetrical about the center transverse axis of the core.

8. In a resistor device,

an elongated core of non-electro-conducting material,

a resistance film on the outer surface of said core,

a helical groove being cut in said film to divide said film into a continuous ribbon comprised of a plurality of convolutions,

the lateral width of the convolutions near the center of said core being wider than the convolutions adjacent the end of said core,

the ends of said groove departing from a helical pattern and terminating in an arcuate shape and extending substantially horizontally and longitudinally towards the respective ends of said core.

9. In a resistor device,

an elongated core of non-electro-conducting material having opposite ends,

a resistance film on the outer surface of said core,

a helical groove having two end portions being cut in said film and having a plurality of convolutions therein extending around said core, with at least one end portion of said groove terminating adjacent one end of said core, said convolutions of said groove dwelling in planes substantially perpendicular to the longitudinal axis of said core,

said one end portion of said groove departing from a helical pattern and terminating in an arcuate shape to extend in a direction towards the adjacent end of said core and to extend in a direction substantially perpendicular to the plane of the adjacent convolution from which said groov departed.

References Cited by the Examiner UNITED STATES PATENTS 1,881,444 10/1932 Flanzer 338-300 X 2,557,983 6/1951 Linder 338-2l7 2,838,639 6/1958 Planer et al 338300 X RICHARD M. WOOD, Primary Examiner. V. Y. MAYEWSKY, Assistant Examiner. 

1. IN A RESISTOR DEVICE, AN ELONGATED CORE OF NON-ELECTRO-CONDUCTING MATERIAL, A RESISTANCE FILM ON THE OUTER SURFACE OF SAID CORE, ELECTRO-CONDUCTING TERMINAL MEANS ON THE ENDS OF SAID CORE IN ELECTRICAL CONTACT WITH SAID FILM, A HELICAL GROOVE BEING CUT IN SAID FILM TO DEVICE SAID FILM INTO A CONTINUOUS RIBBON COMPRISED OF A PLURALITY OF CONVOLUTIONS, THE LATERAL WIDTH OF THE CONVOLUTIONS NEAR THE CENTER OF SAID CORE BEING WIDER THAN THE CONVOLUTIONS ADJACENT THE END OF SAID CORE, THE ENDS OF SAID GROOVE DEPARTING FROM A HELICAL PATTERN AND TERMINATING IN AN ARCUATE SHAPED AND EXTENDING SUBSTANTIALLY HORIZONTALLY AND LONGITUDINALLY TOWARDS THE RESPECTIVE ENDS OF SAID CORE. 